1. Field of the Invention
The present invention relates to an actuating apparatus (actuator) applicable to, for example, an actuation of an electromagnetic valve used for controlling an idling revolution of an internal combustion engine, the valve being installed in an intake air passage and being exposed to an intake air flow.
2. Description of the Related Art
A previously proposed actuator is exemplified by a Japanese Patent Application Second Publication No. Showa 64-2023 published on Jan. 13, 1989.
In the above-identified Japanese Patent Application Second Publication, a rotatable two-pole magnetized permanent magnet is used as a rotor and is rotated within a space (which is long in an axial direction of a first magnetic circuit (path)) is formed within the first magnetic circuit so that a magnetic resistance of a magnetic path generated by the rotating permanent magnet is varied, thus a torque to return the permanent magnet to a normal stable position (in a direction connecting both narrowly formed parts of the space, namely, in a direction of a magnetic flux generated by the permanent magnet orthogonal to an axial direction of the first magnetic path) being acted upon the rotating permanent magnet.
In this state, when a power is supplied to a coil installed in the previously proposed actuating apparatus so as to generate a magnetic flux in the axial direction of the first magnetic circuit which is orthogonal to the magnetic flux generated by the permanent magnet, thus the rotor, i.e., the permanent magnet being rotated from the neutral stable position up to an arbitrary angular position determined according to a magnitude (strength) of the magnetic flux (a magnitude of a power supply current to the coil). Consequently, the rotor can be held at the arbitrary stable angular position.
In addition, in the previously proposed actuating apparatus, another yoke member (a second yoke) made of a ferromagnetic material and which is installed so as to form a second magnetic circuit (path) serves to reduce the magnetic resistance of the magnetic circuit formed by the rotatable permanent magnet at the neutral stable position so as to enlarge the torque to return the permanent magnet to the neutral stable position.
Consequently, a torque variation rate (gradient) with respect to the rotating angle of the rotatable permanent magnet is enlarged. That is to say, a force acted upon the permanent magnet to hold it at its position is enlarged.
However, the previously proposed actuating apparatus disclosed in the above-identified Japanese Patent Application Second Publication has the following problems to be solved.
That is to say, in the previously proposed actuating apparatus, the rotatable two-pole magnetized permanent magnet is arranged as the rotor in the space formed within the yoke member (first yoke) made of the so ferromagnetic material for forming the first magnetic circuit (path).
In details, a whole periphery of a magnetic field formed around the rotatable permanent magnet is short-circuited by means of the member (first yoke) used to form the first magnetic circuit (path).
Part of the magnetic flux generated by the permanent magnet in the direction of the second magnetic path orthogonal to that of the magnetic flux in the first magnetic path is also branched into the first magnetic circuit in which the coil is provided so that the magnetic flux generated in the direction of the first magnetic path orthogonal to the magnetic flux generated by the rotating permanent magnet is reduced (weakened). Therefore, a variation in the magnetic resistance of the magnetic field along with the rotation of the permanent magnet becomes moderate.
Consequently, the torque variation (gradient) with respect to the rotating angle of the permanent magnet is small. In addition, the force acted upon the permanent magnet so as to hold it at its position against an external disturbance torque becomes weak.
Hence, the rotated position of the permanent magnet, namely, the rotor is largely varied according to a magnitude of the external disturbance torque.
For example, in a case where the previously proposed actuating apparatus directly actuates a valve installed in a part of an intake air passage of an internal combustion engine, a torque generated by an external force such as a force generated by an intake air stream (air flow force) is acted upon the rotatable permanent magnet as the rotor in the form of the external disturbance so that the rotated position of the permanent magnet is varied and, accordingly, an opening angle of the valve is varied. Consequently, a hunting occurs in an engine revolution speed.
It is noted that even if the second magnetic circuit is formed by means of the yoke member (second yoke) made of the ferromagnetic material, the whole periphery of the magnetic field generated by the member (first yoke) for forming the first magnetic circuit (path) is short-circuited so that a large improvement of prevention of the variation of the rotated position of the permanent magnet cannot be achieved.
In addition, a split may be provided in the member (first yoke) for forming the first magnetic circuit (path) so as to be divided into two in place of the narrowly formed space described above, thus eliminating the magnetic short-circuit around the permanent magnet.
However, since a first yoke of the first yokes divided into two by means of the split is linked into the first magnetic circuit, the first magnetic circuit itself is linked into the first magnetic circuit, the first magnetic circuit itself forms a magnetically short-circuit route for the permanent magnet. Consequently, the large improvement of prevention of the variation in the rotated position cannot be achieved as well as described above.
Although it is possible to eliminate the above-described problem by enlarging the coil magnetomotive force required to rotate the permanent magnet through the increase in the power supply current to the coil or by using a rare earth element magnet having a strong magnetomotive force for the permanent magnet, the former results in a large power consumption and the latter results in an expensive (increase in cost) actuating apparatus since the rare earth element magnet is very expensive as compared with a ferrite magnet.
It is noted that the ferrite magnet in a cylindrical shape is difficult to be manufactured since cracks are often generated in the ferrite magnet in the cylindrical shape during the manufacturing process.